Prior art devices in this field of which the inventor is aware include those disclosed in the following U.S. patents:
U.S. Pat. No. 5,378,419 PA1 5,206,979 PA1 4,930,179 PA1 4,257,140 PA1 3,669,069 PA1 1,811,372 PA1 1,789,627 PA1 1,700,079 PA1 1,121,272 PA1 589,467
Copies of these patents are readily available in a number of public libraries and from the Patent and Trademark Office, as well as from various sources on the Internet.